1. Field of the Invention
The present invention relates to the technical field of magnetic resonance imaging, in particular to a method and apparatus for compensating for gradient delay in a magnetic resonance scanning sequence.
2. Description of the Prior Art
During a magnetic resonance (MR) scan, an MR scanning apparatus applies a gradient field to the sample being scanned by generating an MR scanning sequence having a gradient waveform. In order to obtain accurate scanning results, the gradient field must be precisely controlled. However, owing to the intrinsic characteristics of the MR scanning apparatus, such as self-inductance from electrical cables or gradient components, gradient delays will arise during generation of the MR scanning sequence, i.e. there will be a time difference between the moment at which a gradient waveform in the MR scanning sequence is actually generated and the ideal moment at which the gradient waveform in the MR scanning sequence is designed to be generated. Thus, it is necessary to compensate for time delays in the MR scanning sequence, to obtain a satisfactory scanned image.
FIG. 1 shows an existing method for performing compensation in an MR scanning sequence. As FIG. 1 shows, the method includes the following steps.
In step 11, multiple gradient delays are measured for an MR scanning apparatus. Generally, gradient delay can be measured at two typical gradient amplitudes. For example, the gradient delay can be measured when the gradient amplitude is 5 milliteslas/meter (mT/m) and 8 mT/m.
In step 12, the average value of the multiple gradient delays is calculated.
In step 13, fixed gradient delay compensation is performed on each gradient axis, the gradient delay used during compensation being the average value calculated. An MR scanning apparatus includes gradient axes in three directions, and in this step, the same gradient delay is used to perform compensation on each gradient axis.
It can be seen that with the existing method, once the gradient delay of the MR scanning apparatus has been determined, the gradient delay will remain unchanged even if the gradient amplitude changes; the effect of gradient amplitude on gradient delay has not been taken into account, and as a result the above method is mainly suited to MR scanning procedures in which the gradient amplitude remains unchanged. However, the development of MR scanning technology has led to there often being a need to adjust the gradient amplitude of the MR scanning sequence, in order to obtain more precise scanning results. Furthermore, high-level sequences such as echo planar imaging (EPI) pulse sequences and ultra-short echo time sequences have exacting requirements in terms of accuracy of gradient delay. In these cases, if gradient delay compensation is performed by the existing method, loss of image quality or artifacts will result, affecting the accuracy of the MR scan.